Amplitude modulation (AM) is one of the best known means of encoding a signal with information to be transmitted between parties, with literally millions of AM compatible receivers being available in the public domain. Amplitude modulation has the advantage of providing good signal propagation due to the low frequency bands used. During emergency situations AM has shown itself to be a highly reliable and practical means for distributing information, particularly in the medium wave (MW) 540-1650 kHz.
In order to improve reliability and decrease transmission overheads in amplitude modulation, modern transmitters use digital switching techniques. Such techniques require high power digital to analogue converters that are based on high-speed multilevel power converters, which switch at the RF carrier frequency. To date, such power converters have been designed based on a conventional arrangement of transformer coupled H-bridge class D amplifiers. An example of such an arrangement is provided in FIG. 1, which shows an AM transmitter 101 for MW, using a digital amplitude modulation (DAM) technique. In this example, the input analogue audio signal is first sampled with a 12 bit digitizer 103, with the 7 most significant bits being de-multiplexed and used to drive 127 identical H-bridge class D switching cells 105 (an example of such a switching cell being shown in FIG. 2). The lower 5 bits are in turn used to drive 5 binary weighted amplifiers 107, which serve to increase the resolution in the output signal. Each one of the 5 binary weighted amplifiers 107 supplies ½ the power of the one above it; so, for example, the first of the 5 binary weighted amplifiers will provide ½ the power of the 127 identical H-bridge class D switching cells 105, and the last of the 5 binary weighted amplifiers will provide 1/32 of the power of the 127 identical H-bridge class D switching cells 105.
Conventional power converters, such as that shown in FIG. 1 have a core limitation that in order to achieve sufficient signal fidelity, it may be necessary to include as many as 132 H-bridge cells, with each H-bridge cell requiring its own transformer to couple the cell to the output. As a result of using this number of transformers, the weight, size and manufacturing costs of the converters are increased.